Try and fix fernflower forking for the final time. Hopefully helps with #355

src/main/java/net/fabricmc/loom/decompilers/fernflower/AbstractFernFlowerDecompiler.java

@@ -100,7 +100,7 @@ public abstract class AbstractFernFlowerDecompiler implements LoomDecompiler {
 
 		progressGroup.started();
 		ExecResult result = ForkingJavaExec.javaexec(
-				project.getRootProject().getPlugins().hasPlugin("fabric-loom") ? project.getRootProject() : project,
+				project,
 				spec -> {
 					spec.setMain(fernFlowerExecutor().getName());
 					spec.jvmArgs("-Xms200m", "-Xmx3G");

src/main/java/net/fabricmc/loom/decompilers/fernflower/ForkingJavaExec.java

@@ -24,11 +24,10 @@
 
 package net.fabricmc.loom.decompilers.fernflower;
 
+import java.net.URLClassLoader;
+
 import org.gradle.api.Action;
 import org.gradle.api.Project;
-import org.gradle.api.artifacts.ConfigurationContainer;
-import org.gradle.api.artifacts.dsl.DependencyHandler;
-import org.gradle.api.file.FileCollection;
 import org.gradle.process.ExecResult;
 import org.gradle.process.JavaExecSpec;
 
@@ -40,13 +39,8 @@ import org.gradle.process.JavaExecSpec;
  */
 public class ForkingJavaExec {
 	public static ExecResult javaexec(Project project, Action<? super JavaExecSpec> action) {
-		ConfigurationContainer configurations = project.getBuildscript().getConfigurations();
-		DependencyHandler handler = project.getDependencies();
-		FileCollection classpath = configurations.getByName("classpath")//
-						.plus(configurations.detachedConfiguration(handler.localGroovy()));
-
 		return project.javaexec(spec -> {
-			spec.classpath(classpath);
+			spec.classpath((Object[]) ((URLClassLoader) ForkingJavaExec.class.getClassLoader()).getURLs());
 			action.execute(spec);
 		});
 	}

src/test/groovy/net/fabricmc/loom/DecompileTest.groovy

@@ -25,30 +25,30 @@
 package net.fabricmc.loom
 
 import net.fabricmc.loom.util.ProjectTestTrait
-import spock.lang.Ignore
 import spock.lang.Specification
 import spock.lang.Unroll
 
 import static org.gradle.testkit.runner.TaskOutcome.SUCCESS
 
-@Ignore // TODO this fails due to the plugin classpath being setup differently. We need to explore other ways to do this.
 class DecompileTest extends Specification implements ProjectTestTrait {
 	@Override
 	String name() {
-		"simple"
+		"decompile"
 	}
 
 	@Unroll
-	def "#decompiler decompile"() {
+	def "#decompiler gradle #gradle"() {
 		when:
-			def result = create(task)
+			def result = create(task, gradle)
 
 		then:
 			result.task(":${task}").outcome == SUCCESS
 
 		where:
-			decompiler 		| task
-			'fernflower'	| "genSources"
-			'cfr' 			| "genSourcesWithExperimentalCfr"
+			decompiler 		| task								| gradle
+			'fernflower'	| "genSources"						| "6.8.3"
+			'fernflower'	| "genSources"						| "4.9"
+			'fernflower'	| "genSources"						| "7.0-milestone-2"
+			'cfr' 			| "genSourcesWithExperimentalCfr"	| "6.8.3"
 	}
 }

src/test/groovy/net/fabricmc/loom/LegacyProjectTest.groovy

@@ -44,10 +44,4 @@ class LegacyProjectTest extends Specification implements ProjectTestTrait {
 		then:
 			result.task(":build").outcome == SUCCESS
 	}
-
-	@Override
-	String warningMode() {
-		// Gradle 4.9 doesnt support fail, and well this is a legacy test so deprecations are expected
-		'all'
-	}
 }

src/test/groovy/net/fabricmc/loom/util/ProjectTestTrait.groovy

@@ -75,7 +75,7 @@ trait ProjectTestTrait {
 	BuildResult create(String task, String gradleVersion = "6.8.3") {
 		GradleRunner.create()
 			.withProjectDir(testProjectDir)
-			.withArguments(task, "--stacktrace", "--warning-mode", warningMode(), "--gradle-user-home", gradleHomeDirectory(gradleVersion))
+			.withArguments(task, "--stacktrace", "--warning-mode", warningMode(gradleVersion), "--gradle-user-home", gradleHomeDirectory(gradleVersion))
 			.withPluginClasspath()
 			.withGradleVersion(gradleVersion)
 			.forwardOutput()
@@ -83,7 +83,11 @@ trait ProjectTestTrait {
 			.build()
 	}
 
-	String warningMode() {
+	String warningMode(String gradleVersion) {
+		if (gradleVersion == "4.9") {
+			return "all"
+		}
+
 		System.getenv().TEST_WARNING_MODE ?: 'all'
 	}
 

src/test/resources/projects/decompile/build.gradle

@@ -0,0 +1,9 @@
+plugins {
+	id 'fabric-loom'
+}
+
+dependencies {
+	minecraft "com.mojang:minecraft:1.16.5"
+	mappings "net.fabricmc:yarn:1.16.5+build.5:v2"
+	modImplementation "net.fabricmc:fabric-loader:0.11.2"
+}